The present invention relates to a device for measuring electromagnetic interference (EMI), of the type comprising a closed test cell the periphery of which is defined by an outer conductor of a waveguide constituted by a tubular casing having rectangular cross-sections and delimited at one end by a supply cusp and at the other end by a transverse wall which is absorbent as regards high frequencies and is associated with resistors for low-frequency dissipation, and an inner conductor of the waveguide extending within the cell between the supply cusp and the resistors.
A device of this type is known from U.S. Pat. No. 4,837,581 amongst others.
Other similar devices are known, for example, from the following publications: IEEE Transactions on Electromagnetic Compatibility, Vol. EMC-16, No. 4, November 1974, pp. 189-195, Pollard, N., "A Broad-Band Electromagnetic Environments Simulator (EMES)", IEEE Symposium on EMC, Seattle, August 1977, IEEE 1978 International Symposium on EMC-Symp. Rec. (Atlanta, Ga.), June 1978, pp 204-208.
In all these known devices, the inner conductor is in the form of a flat plate enclosed by an outer conductor of rectangular or square cross-section.
FIG. 1 is a schematic cross-section of a device for measuring electromagnetic interference according to the prior art. The section is taken in a region which accommodates the object under test.
A tubular casing which constitutes the outer conductor of a TEM waveguide is indicated 10 and the inner conductor of the guide, which is in the form of a flat plate, is indicated 12.
One of the requirements for a measuring device of the type in question is that the intensity of the electromagnetic field in the test region must be substantially uniform.
FIG. 1 shows the lines of equal intensity of the electromagnetic field with respect to a reference intensity, indicated 0 dB.
The reference line 0 dB is located between corresponding lines with relative intensities of +1 dB and -1 dB. The intensity of the field may be considered sufficiently uniform for the purposes of electromagnetic interference tests if the object under test is in a region, indicated Z in FIG. 1, between +1 dB and -1 dB.
In many applications it would be desirable for the region Z to be much larger for given dimensions and proportions of the two conductors 10 and 12.